They both automate our tasks, they both use Node, and they both require you to create tasks and install plugins.

Grunt handles this using intermediary files which are disk I/O operations. Your SASS file is compiled and then written to a temporary file. The temporary file is used by the autoprefixer and then the final product is written to the destination file.

Gulp takes care of all this in-memory. Your source SASS file is compiled, the result is passed to the autoprefixer without being written to a file and the destination file is then written out.
Compared to in-memory operations, disk writes are slow which means that Gulp has a big speed advantage (for now).